Many modern applications, of both the stationary and mobile type, are operated by an electrical current. If a permanent current source in the form of the electricity supply grid or a generator is not available, energy stores for storing electrical energy must be utilized to operate these devices. Particularly effective in this regard are rechargeable stores for electrical energy having an electrochemical basis, i.e. the type that can both store electrical energy and release electrical energy again as necessary. Such a store is therefore an embodiment of a galvanic cell, and is also referred to as a rechargeable battery or an accumulator. The so-called high-temperature metal/metal oxide storage device is a particular type of an electrochemical energy storage device.
Such a high-temperature metal/metal oxide storage device consists of at least one module having two electrodes and, disposed therebetween, an oxygen-conducting electrolyte, as is schematically illustrated in FIG. 1. The legend characters in FIG. 1 have the following meanings:    1) housing or interconnectors;    2) air chamber, in which an oxygen-rich gas (e.g. air) is located;    3) air electrode;    4) oxygen-ion conducting electrolyte;    5) fuel electrode;    6) fuel chamber, in which the so-called shuttle gas (e.g. H2+H2O) is located; and    7) electrochemical storage material.
The components, the air electrode (3), the electrolyte (4), and the fuel electrode (5), form the electrochemical cell. The electrochemical cell can be operated in two different operating modes.
During the discharging of the storage device, the electrochemical cell uses the hydrogen present in the fuel chamber and the oxygen present in the air chamber for energy generation, in a manner similar to that of a fuel cell. Water forms in the fuel chamber as a by-product, which reacts with the electrochemical storage material (7). During this reaction, the oxygen from the water is chemically bound in the storage material via oxidation and hydrogen is released. The released hydrogen is further used as fuel for energy generation. The aforementioned reaction continues to run until all the storage material has been converted or oxidized.
During the charging of the storage device, the water present in the combustion chamber of the electrochemical cell is decomposed with the aid of the electric current. The oxygen from the water is transported, in the form of oxygen ions, by the electrolyte of the cell into the air chamber and, there, is released as gas and loses electrons. On the other side, the hydrogen from the water reacts, accepting electrons, to form hydrogen, which migrates into the storage material and, there, effects a reduction of the oxide that is present. The oxygen from the storage material is taken up and water forms as the product of the reduction process, which migrates back into the fuel chamber.
The H2/H2O gas mixture in the fuel chamber therefore functions as an oxygen transfer agent (shuttle) and allows oxygen transport between the cell and the storage material. It is also possible to use a mixture of CO/CO2 as the shuttle gas, instead of H2/H2O.
A few materials for the electrochemical storage of energy in the form of oxygen are already known from the literature, in particular so-called metal/metal oxide materials are used as energy stores for this purpose. Manganese, iron or nickel, or their alloys, for example, are mentioned as storage materials, which are present in reduced form as metal, and in oxidized form as metal oxides.
With these storage materials, excess electrical energy (such as from wind or solar energy) can be used to reduce the metal oxide (e.g. Fe2O3/Fe3O4/FeO or NiO). The storage device is thereby charged. The reduced storage material can be oxidized as necessary, whereby electrical energy is released again. The store is discharged.
It has been shown, however, that these metal/metal oxide materials (e.g. iron/iron oxide, manganese/manganese oxide or nickel/nickel oxide), which are known as storage materials, do not have adequate long-term stability in the form that has been available to date. This means that, after a large number of discharging and charging cycles, the storage material loses its capability to take up and release energy, or this capability is greatly diminished (degradation). It has been shown that this is due to the insufficient material stability.
In the case of oxide-based storage materials, the discharging and charging cycles in the case of the known metal/metal oxide materials regularly result in separation and coarsening of the storage material. As a result of the two aforementioned processes, a layer consisting of a reactive oxide forms on the surface of the storage material. During the reduction process (battery charging), the oxide layer gradually transforms into a gas-tight metal layer. The formation of such a gas-tight metal layer, however, disadvantageously diminishes or prevents the gas exchange by the storage material with the atmosphere, and thereby prevents further charging processes. The three processes—separation, coarsening, and formation of the gas-tight metallic surface layer—are responsible for the tendency for conventional storage materials to gradually lose effectiveness.
The aforementioned processes are the reason for the disadvantageous agglomeration, which, in turn, results in a reduced active surface (see FIG. 2). In the schematic illustration of the storage material in FIG. 2, the reference characters mean: 1=inert oxide; 2=reactive oxide; 3=surface cover layer consisting of oxide or metal. The starting state of the storage material is shown on the left side (FIG. 2a), in which the inert oxide is present so as to be virtually evenly distributed in the reactive oxide. The state after multiple charging and discharging cycles is shown on the right side (FIG. 2b). The inert oxide has agglomerated and a cover layer consisting of oxide and metal has formed on the surface, which prevents the further charging and discharging cycles.
Attempts were made in the past to prevent this reaction of known metal/metal oxide materials, which was perceived to be disadvantageous. To this end, attempts were made, for example, to prevent agglomeration or coarsening of the material using different additions of chemically inert oxides. Zirconium dioxide doped with yttrium, as the chemically inert oxide, for example, was used as the ionic conductor or the support matrix. Oxides such as cerium oxide or titanium dioxide were used in small quantities as the ceramic strengthening phase for material formation, and were intended to result in so-called ODS (oxide dispersion strengthened) materials. It was shown that agglomeration or coarsening of the material could be slowed, due to longer diffusion paths, but not prevented, at the expense of the energy density.
The basic mode of operation of a storage device for storing or releasing chemically stored electrical energy is on the basis of two different examples of the known metal/metal oxide (Me/MeOx) store, in particular a Fe/(Mg,Fe)O store, which is operated with oxygen, i.e. for example with an H2O/H2 or CO2/CO gas mixture.
FIGS. 3a and 3b each show, at different scales, the microstructure of the storage material Fe/(Mg,Fe)O with an inert oxide (ZrO2) after 10 cycles, wherein the last cycle was a charging cycle.
By comparison, FIGS. 4a and 4b each show, also at different scales, the microstructure of the same storage material with the same inert oxide after 11 cycles, in this case the last cycle being a discharging cycle.
In FIGS. 3 and 4, the reference characters mean: 1=storage material, 2=embedding mass (black), auxiliary material for the preparation of the polished specimens, i.e. not present in the battery operation, 3=oxide or metal layer (light gray), 4=inert oxide (dark gray), and 5=Fe oxide, or Fe.
In the case of storage materials based on a metal alloy, an effect also occurs wherein a closed metal layer disadvantageous forms during the charging cycle, which prevents charging processes, after a relatively short time (see FIG. 5).
In FIG. 5, the reference characters mean: 1=storage material (metal), 2=embedding mass (black), 3=oxide layer, and 4=closed metal layer.
Furthermore, an increase in the porosity, due to the addition of pore-forming material such as polymers, graphite, or starch, was investigated, which was likewise capable of delaying, but not preventing, the agglomeration/coarsening and the formation of the outer layer.
The formation of chemically stable mixed oxides, such as chromite spinels, also did not achieve the objective in terms of avoiding structure coarsening while simultaneously retaining reversibility. The reason for this, inter alia, is that the thermodynamically formed mixed oxides in the selected reduction-oxygen partial pressure range are no longer reducible or the reduction kinetics are greatly slowed.